1. Field of the Invention
The present invention relates to an optical wireless communication method and apparatus.
The invention is particularly concerned with a method and an apparatus for constructing a local area network (LAN) using optical transmission in space.
2. Description of the Prior Art
Local area networks are employed not only in offices but factories or in the open air for the data communications, because the optical LANs are unaffected by electromagnetic noises.
In the LANs called Ethernet (registered trade mark), a code called a header is appended at the front of the original data to be transmitted. In the header, "10101010" is repeated seven times (seven bytes) and "10101011" indicating the start of the original data is transmitted one time (one byte). Thereafter, the original data are sent out.
There are shown in FIG. 1 internal elements of a prior art optical wireless communication apparatus.
In FIG. 1, there are an optical receiving window 303 to receive optical signals, an optical transmitting window 304 from which the optical signal is transmitted and a connector opening 305. There are printed circuit boards 308 and 310 inside a housing 302.
A photo diode PD and many light emitting diodes LEDs are mounted on the printed circuit board 308. A connector 311 and many parts are mounted on the printed circuit board 310. The optical wireless communication apparatus 300 transmits and receives optical signals of the Manchester code by using the photo diode PD and light emitting diodes LEDs.
There is shown in FIG. 2 an optical wireless communication apparatus 300 connected with a personal computer 320 by a connection cable 315. The personal computer 320 is communicatable with the others by using the optical wireless communication apparatus 300. In the data communication between the apparatus 300 and the computer 320, the Manchester code is used in the same way as the data communication in space.
There are shown in FIG. 3 many pairs of personal computers 320 and optical wireless communication apparatuses 300. The computer 320 can communicate with each other via a satellite 316 fixed on the ceiling 317. Therein the Manchester code is used for multi-accessible data communications.
Many pairs of personal computers 320 and optical communication apparatuses 300 make up an optical wireless LAN. The computers 320 can communicate with each other using the Manchester code.
There are shown in FIG. 4 several kinds of codes. In FIG. 4, (a) indicates an original data to be transmitted to a receiver. FIG. 4 (b) denotes the data encoded from the original data of (a) to the Manchester code called the dipulse code. In the same manner, (c) represents the data encoded from original data of (a) to the NRZ (i.e., nonreturn-to-zero) code, and (d) designates the data encoded from the original data of (a) to the DMI (i.e., differential mode inversion) code.
Each of codes has merits and demerits. The Manchester code is used in the communications between optical wireless communication apparatuses 300 and personal computers 320 in FIG. 3, too, because synchronizing signals are easily detectable. The spectrum of the NRZ code spreads in lower frequency ranges than that of the Manchester or the DMI code. Therefore, the NRZ code is suitable for use in internal circuits of the apparatus. However, the NRZ code is unsuitable for AC amplifying, because the NRZ code has the DC ingredient.
As the DMI and Manchester codes have no DC ingredient, the codes are suitable for AC amplifying. However, when many "1s" or "0s" continue in the DMI and Manchester codes, it is difficult to obtain synchronization signals. Therefore, in case of employing the DMI or Manchester code, some measures are needed to protect occurrences of unsynchronized states. As the Manchester code has no redundancy in its construction, it is difficult to obtain functions of the transmission line watch to prevent occurrences of error data.
In the LAN of FIG. 3, the Manchester code of 10 Mbps, for example, is employed as the optical wireless signals. Therein if one of optical wireless communication apparatuses 300s has high optical receiving sensitivity, the apparatus 300 detects incorrect data by receiving external optical noises in spite of no LAN signal.
Receiving incorrect data, the apparatus 300 can not transmit any optical signal because of the collision avoidance function. As the apparatus 300 has to try the transmission again after no existence of the incorrect data, it is impossible to obtain the high communication efficiency.
Therefore, the optical communications are executable by employing low sensitive optical receivers not to operate the collision avoidance function. By the employing such receivers, wide enough optical LAN service area is unobtainable.
As the number of apparatuses included in the optical LAN service area is reduced by the scale-down of the service area, it is unexpected to obtain the high efficiency and excellent optical data communications.
Let it be supposed that the optical stable communicable distance in a straight line i.e., the mutual distance between optical apparatuses is 10 meters, for instance, the optical energy is reflected at an attenuation rate of 10 percent from the ceiling of the reflection coefficient of 10 percent.
Therefore, the transmittable distance of about 3 meters being the square root of 10 meters is obtained. The distance between the apparatus and the ceiling is limited within 1.5 meters accordingly. The optical apparatus of the communicable straight line distance of 30 meters has to be employed to obtain the effective transmittable distance of 10 meters using the reflection of the ceiling. The directivity of an angle of about +/-20 degrees is needed to obtain some extent of communicable area.
In view of such a situation, however, the optical apparatuses having the directivity of the angle of +/-1 degree, for example, are used to obtain the long distance communications. As those have the sharp optical directivity, the multi-access operation is unexpected. The communication is limited between the specific two apparatuses.
In the optical LAN of FIG. 3, when many apparatuses 300s transmit optical signals simultaneously, the satellite 316 receives collided optical signals. Not detecting the ordinary optical signal, the satellite 316 transmits the collided optical signals to many apparatuses 300 as the received signals. It is therefore, difficult to avoid occurrences of collisions, unless the communication efficiencies are remarkably reduced by the sacrifice of the data bit rate.
In the optical LAN of FIG. 3, the communications use the Manchester code of 10 Mbps. The communications can be occasionally interrupted by the collision avoidance function caused by optical noises, because of a communicable distance that is too short or an optical receiving sensitivity that is too high. It is thereby unexpectable to obtain high efficient optical communications.
In such a case, the measures that the transmission rate of 10 Mbps is reduced to 1 Mbps are executable to get a higher S/N (signal to noise ratio). As the transmission rate is temporally reduced, big capacity memories and complicated controls are needed. They cause high cost.
In a LAN system, each of optical apparatuses granted addresses can efficiently receive only optical frames appended with self-address. The LAN system is disclosed in Japanese Patent Provisional Publication No. 7-23047.
In another LAN system, a network hierarchy includes repeaters connected with optical LANs. The optical LAN frames are appended with hierarchy addresses. The hierarchy system is disclosed in Japanese Patent Provisional Publication No. 7-123052.